Downregulation of Alloimmunity in Lung Allograft by Eosinophils is Orchestrated by PD-L1 Dependent Contact with CD8+ T Cells

Abstract

Purpose Eosinophils play a surprising role in lung allograft-specific tolerance induction dependent on the blockade of the CD40 and CD28 stimulatory pathways (CSB). The mechanisms used by eosinophils to downregulate the alloimmune response remains elusive. Methods To study the mechanism of eosinophil-dependent immune responses fully MHC-mismatched Balb/c (H2Kd) lungs were engrafted to C57BL/6J (H2Kb) recipients without immunosuppression. For some experiments eosinophils were ablated through the administration of diphtheria toxin (DT) to C57BL/6 mice expressing DTR on an eosinophil-peroxidase specific promoter (iPHIL mice). To model alloreactivity in vitro, mixed leukocyte cultures, comprising Th1-polarized (E1) eosinophils as modulators of T cells from C57BL/6J mice stimulated by either allogeneic dendritic cells or anti-CD3/CD28 agonistic antibodies. Results Lung graft-resident eosinophils demonstrated a Th1 polarized phenotype (or E1 polarization) in the presence or absence of CSB. In the absence of CSB eosinophils express higher levels of MHC-1 and PD-L1. Ablation of eosinophils in non-immunosuppressed graft recipients enhances T cell activation with higher ISHLT rejection grade (p Conclusion We demonstrate that even without immunosuppression, eosinophils contribute to the downregulation of alloimmunity. Eosinophils upregulate PD-L1 and use this mechanism to mediate contact-dependent downregulation of T cell activation. We also show that mobilization of eosinophils into the lung allograft through intra-tracheal instillation of eotaxins and IL-5 can be utilized to downregulate the alloimmune response and sets the stage for unique and previously unexplored strategies for tolerance induction.